1. Field of the Invention
The invention relates to a device for controlling the temperatures in a cooling chamber for microtomes, in particular ultramicrotomes, having at least two different temperature control circuits for controlling the temperatures of the specimen and of the cutter.
2. Related Prior Art
The cutting of soft, elastic or hard samples, or, in very general terms, of materials whose consistency does not allow them to be cut at room temperature (e.g. aqueous suspensions, paints) requires cooling to temperatures which in some cases are well below room temperature. In this case, special cooling chambers are used for the microscopic, in particular electron-microscopic examination, and these cooling chambers, combined with suitable microtomes or ultramicrotomes, make it possible to remove sufficiently thin sections (cf. in this respect, inter alia, H. Sitte, Ultramikrotomie, in: mta-journal extra No. 10, Umschau-Yerlag, Breidenstein GmbH, Frankfurt-Main, 1983). The temperature range of cooling chambers of this nature is between room temperature and approximately -180.degree. C., and it is possible to control the temperatures of the specimen and of the cutter, and in many cases also of the chamber gas, separately. This separate control of the three important temperatures has long proven advantageous, since in this way it has been possible for all the thermal parameters which influence the cutting operation to be optimized separately. The cooling medium ("cryogen") used was in most cases either cold nitrogen gas (N.sub.2g) from a Dewar vessel filled with liquid nitrogen (N.sub.2l) or N.sub.2l for direct cooling. The gas which is present in the cooling chamber and floats around the cutter and specimen in these cases consists of pure N.sub.2g.
The preselected temperatures of specimen, cutter and chamber gas were reached and kept constant in a technically known way using electronic control circuits which each have a temperature sensor (e.g. Pt 100 platinum resistor or microthermocouple) and a heater element (e.g. heating resistor). Setting elements allowed the desired temperatures to be preselected, and temperature indicators allowed the values reached in each case to be checked. If one takes into account the fact that the cutter temperature is the decisive factor for, for example, the use of certain float liquids (e.g. DMSO/H.sub.2 O), whereas the specimen temperature determines the consistency of the specimen (e.g. brittle or ductile, or liquid or solid), it will be understood that this type of separate temperature control of the three essential temperatures has without exception been felt to be optimum, and that cooling chambers of a very wide variety of designs and manufacturers were always equipped with the separate temperature control for specimen and cutter, and sometimes also for the chamber gas, which has been described.
In everyday practice, however, this principle exhibits considerable drawbacks and shortcomings which stem from the technical options available for precise temperature measurement in extremely small areas. Currently, it is technically impossible to measure the significant temperatures correctly. Even the smallest temperature sensors which operate virtually without inertia (e.g. microthermocouples) cannot be integrated in the outermost surface layers of the specimen. The specimen temperature always has to be measured at a lower layer which is, for example, at a distance of from 0.5 to 1 mm from the surface of the specimen and therefore from the location of the cutting process. The same applies to the cutting edge. It is not possible to arrange sensors at the actual cutting edge either in the diamond cutters which are preferably used nowadays in ultramicrotomy or in the less expensive metal or glass blades.
For practical reasons, the tendency is to position these sensors in the compact metal holders of the specimen and cutter in order to allow cutter and specimen to be replaced frequently in a simple and rapid manner. In many cases, the sensors arranged on these components show temperature values which are far removed from reality. This makes it almost impossible to reproduce the temperatures, particularly if cooling chambers of different designs or from different manufacturers are used.
The situation becomes particularly problematical if different temperatures are preselected for the specimen, cutter and chamber gas. It is known that a cold chamber gas cools the surfaces of cutter and specimen to well below the preselected or indicated values. Conversely, the thermal radiation from the warmer cutter onto the cooler specimen brings about significant warming of the surface layers of the specimen. It is scarcely possible to state the extent to which the indicated temperatures are a distortion of the true temperatures, and there is always a state of considerable uncertainty. This uncertainty is compounded by the dynamic phenomena which it is necessary to expect in view of the equilibria of flow, which often establish themselves slowly. Finally, attempting to achieve optimum results by systematically changing the three said temperatures is virtually pointless in practice, since the combination of equipment allows a virtually endless number of combinations. In any case, once an optimum temperature combination has been determined, this cannot readily be transferred to other cooling chambers, and in most cases is therefore of only very low systematic value.
Apart from these essential problems, it is necessary to carry out two, usually three or in special cases even more setting operations in order to preselect the temperatures which are desired in each case. Also, it is necessary to be convinced that all the preselected values have actually been reached before the sectioning is started after the temperature has been preselected or changed or before the sectioning is continued after the values have recently been changed. All in all, the result is a not insignificant delay in practical work for the majority of sectioning processes at reduced temperature.